1. Field of the Invention
The present invention relates, in general, to low pressure gas discharge lamps. In particular, the invention relates to a flat type low pressure gas discharge lamp which is applied to a liquid crystal display (LCD) apparatus, as a backlighting.
2. Description of the Related Art
As is well known, a low pressure gas discharge lamp, e.g., a fluorescent lamp, is used as a light source for a liquid crystal display apparatus. Essential features required as a light source for a liquid crystal display are a reduced thickness, a high brightness, and a uniformity of brightness.
Japanese Laid-open patent 62-208537 discloses a flat type fluorescent lamp which satisfies the above-described requirements. However, in such a flat type fluorescent lamp disclosed in the Japanese laid-open patent, when an aspect ratio F defined by the ratio between the elongated width W which is parallel to the electrode discharge surface and the height H of the flat type fluorescent lamp is greater than a prescribed value, undesirable phenomena, such as, a discharge concentration, a fluctuating positive column, etc., occur, resulting in unstable operation of the flat type fluorescent lamp.
The British Journal of Applied Physics, Vol. 11 (1960) 492498, authored by M. A. Cayless discloses the discharge concentration phenomenon in which discharge of the flat type fluorescent lamp concentrates as the above-described aspect ratio F increases. According to this Journal, the upper limit value of the aspect ratio F is 3.5.about.4, and, in particular, the discharge of the lamp is stable under a normal current density in the discharge space only when the aspect ratio F is less than 3.5. In addition, it has been considered that it is difficult to achieve stable operation when the aspect ratio F is more than 4 even if the current density of the discharge space is varied greatly.
It is preferable to increase the light radiating surface of the flat type fluorescent lamp when it is used as a light source for the liquid crystal display apparatus. It is also preferable to reduce the thickness (height H) of the lamp to achieve a desirable external shape of the liquid crystal display when it is used in the display apparatus. However, since the upper limit of the aspect ratio F of the lamp is limited, as described above, a plurality of flat type fluorescent lamps may be needed if the above-described flat type fluorescent lamp having a relatively small light radiating surface is applied to a relatively large liquid crystal display apparatus. Thus, the number of areas between the plurality of flat type fluorescent lamps increases resulting in irregularity of brightness of the display apparatus. In addition, increase in the number of flat type fluorescent lamps used in the display apparatus causes an increase in the number of lead elements requiring a large supporting space in the display apparatus. Thus, the application of the prior art design to LCDs may be limited, and the cost of manufacturing the display apparatus is high.